Can somebody tell me why my kitchen tap leaks after use or when the upstairs toilet is flushed?

I changed a stop cock that broke and had to isolate the water from outside to carry out the repair. Ever since I did that the problem has occurred. It seems like I need to set the pressure from the outside tap which I have tried without success

Help please !

Do you know what your water pressure is?

Sorry no I don't - Just have to keep playing around with it - adjusting the tap outside and inside to try and solve.

Hot water is from Combi Boiler

Maybe we are mixing our terms here. To most of us a tap is a faucet or spigot, you can not alter water pressure or anything by using a tap/spigot. Can you better explain what you are doing "adjusting the tap outside and inside to try and solve"

Sorry outside and inside tap I mean stop cock